This invention relates generally to an electrophotographic printing machine, and more particularly concerns an optical system having an opaque member with an array of apertures therein associated with a lens to improve the light transmission thereof.
A typical electrophotographic printing machine exposes a charged photoconductive member to a light image of an original document. The irradiated areas of the photoconductive member are discharged selectively recording thereon an electrostatic latent image corresponding to the original document being reproduced. A development system moves a developer mix of carrier granules and toner particles into contact with the latent image recorded on the photoconductive member. The toner particles are attracted electrostatically from the carrier granules to the latent image. In this manner, a powder image is formed on the photoconductive member. Thereafter, the powder image is transferred to a sheet of support material. After transfer, the sheet of support material passes through a fusing device which permanently affixes the toner powder image thereto.
In multi-color electrophotographic printing, the foregoing process is repeated a plurality of cycles for each discrete color contained within the original document. Hence, multi-color printing requires the light image to be filtered to record an electrostatic latent image on the photoconductive member corresponding to a single color of the original document. This single color electrostatic latent image is developed with toner particles of a color complementary to the color of the filtered light image. Thereafter, the toner powder image is transferred to a sheet of support material. This process is repeated for successively differently colored light images. Each toner powder image is transferred, in superimposed registration with the prior toner powder image, onto the sheet of support material. In this manner, a multi-layered toner powder image is formed on the sheet of support material containing therein the colors of the original document. This multi-layered toner powder image is then affixed permanently to the sheet of support material forming a permanent color copy of the original document.
In most electrophotographic printing machines, tone graduations are difficult to form. This problem may be obviated by the utilization of a screening technique. Generally, screening methods produce the effect of tone graduations by variations of the diameter of the half-tone dots or the width of the half-tone lines comprising the toner powder image created by the screen. In the highlighted zones or regions of high intensity of illumination, the dots or lines are small, increasing in size through the intermediate shades until they merge together in the shadow regions. At the highlight end of the tone scale, there will be complete whiteness while at the shadow end, nearly solid blackness. The foregoing is described more fully in U.S. Pat. No. 2,598,732 issued to Walkup in 1952. Other patents exemplifying various screening techniques are U.S. Pat. No. 3,535,036 issued to Starkweather in 1970; U.S. Pat. No. 3,121,010 issued to Johnson et al. in 1964; U.S. Pat. No. 3,493,381 issued to Maurer in 1970; U.S. Pat. No. 3,776,633 issued to Frosch in 1973; and U.S. Pat. No. 3,809,555 issued to Marley in 1974.
It is well known, with the screen interposed between the lens and photoconductive surface, that the lens aperture size must often be restricted to obtain the desired aerial image modulation behind the screen. This is due to the light shadow-refraction pattern behind the screen being determined by the solid angle of the light rays incident on the screen. Optimization of the lens to achieve the desired aerial image modulation often reduces the overall light intensity transmitted by the lens relative to that which may be transmitted when a clear aperture is employed. This results in a more intense light source being required to obtain the same exposure level. This problem is especially acute when "phase" screens are employed. In a typical phase screen system having a 1:1 magnification, this may require an effective F-stop of about 50 compared with the typical F-stop of about 5.6 to 4.5 presently employed in electrophotographic printing machines. Contrawise, a typical ruled screen may require an F-stop of anywhere from 6.3 to 11. Thus, it is evident that there is significant light loss in either of the foregoing cases.
Multi-aperture stops have been employed in photography. For example, A. Freuwirth in American Photoengraver, 28, 275 (1936) discloses work performed in 1894. As described therein, a lens stop could be designated with two, four, or more apertures suitably distributed over a lens for use with half-tone photography. Additionally, U.S. Pat. Nos. 2,703,281 and 2,920,547 issued to Consaul et al. disclose a lens aperture plate having an array of openings therein. The openings relate in size to the optimum F-stop and in geometry to the half-tone screen. Various other prior art references teach similar types of processes, exemplary of these are U.S. Pat. Nos. 1,460,744 issued to Boysen; 2,145,427 issued to Morris, Jr.; 2,478,443 issued to Yule et al.; 2,959,105 issued to Kazuo Sayanagi; and 3,340,061 issued to McCarthy.
It is a primary object of the present invention to improve electrophotographic printing by employing an aperture array associated with a lens for use in a half-tone imaging system in electrophotographic printing wherein the light intensity transmitted to the photoconductive member is optimized.